For example, when a beam of electrons or X-rays is focused on a sample surface to analyze an element, secondary electrons such as excited Auger electrons or photoelectrons are emitted. The energy of these secondary electrons easily changes as the potential of the sample surface changes. Such changes in the potential of the sample surface are observed when the sample surface is charged by a beam of electrons from an excitation source or by an emission of secondary electrons. With a conventional method, changes in the potential on the sample surface are restrained by preventing the sample surface from being charged.
For example, the following methods are used to prevent the sample surface from being charged when a beam having charges is applied to the sample surface from the excitation source.
(1) Incident charges and emitted charges are caused to balance by varying the energy, incident angle and amount of current of a beam applied to the sample surface from the excitation source. PA1 (2) Positive charges and negative charges on the sample surface are caused to balance by simultaneously applying to the sample a beam from the excitation source and a beam having a charge opposite to that of the beam from the excitation source. PA1 (3) The amount of charges per unit area on the sample surface is reduced by decreasing the current density of the excitation source. PA1 (4) If the sample has a multi-layer structure including an electrically conductive layer as a lower layer, the energy of the beam is increased to cause the beam from the excitation source to reach the electrically conductive layer, and the charges are emitted through the electrically conductive layer. PA1 (1) a first beam source for causing a sample to emit analytical secondary electrons by applying a main beam to the sample; PA1 (2) spectrum detecting means for detecting an electron spectroscopy spectrum by analyzing an energy of the analytical secondary electrons; PA1 (3) a second beam source for applying to the sample a charging beam having an energy lower than an energy of the main beam; and PA1 (4) shift detecting means for detecting an amount of an energy shift of a reflected beam produced by applying the charging beam to the sample.
The sample surface is positively charged by the emission of the secondary electrons. Then, a beam (generally an electron beam) having opposite charges is applied to the sample surface so as to cause the positive charges and the negative charges on the sample surface to balance.
However, these methods have the following drawbacks.
These methods are the method of balancing the incident charges and emitted charges, and the method of reducing the amount of charges. They are thus useful to reduce the charges on the sample surface to a level which makes a measurement unavailable. However, they cannot completely prevent a slight change in the potential of the sample surface during the analysis. Namely, they cannot fully prevent a shift of spectral line caused by a slight change in the potential of the sample surface with the passage of time.
For example, with an electron spectroscopy in which the S/N ratio is improved by the addition of data, when a shift of spectral line is caused (see FIG. 5(a)) by the passage of time during the analysis, the analytical spectrum obtained by the addition of data has a broader peak as shown in FIG. 5(b). The spectrum having the peak shown in FIG. 5(d) is expected to be obtained by the addition of the analytical spectra obtained at time t.sub.0, t.sub.1 (or t.sub.0 +.DELTA.t), and t.sub.2 (or t.sub.1 +.DELTA.t) shown in FIG. 5(c). However, when a shift of the spectral line occurs with the passage of time as shown in FIG. 5(a), the peak becomes broader as shown in FIG. 5(b). Namely, when the peak becomes broader due to such a shift, the energy resolution and the accuracy of determination are lowered.
When sputter etching is used for the electron spectroscopy, not only the surface of a sample but also the depth profile thereof is analyzed. In this case, if the sample is a thin film or has a multi-layer structure, the film thickness of the topmost thin film and the constituent elements/composition of the topmost layer change as the sputter etching proceeds. Thus, the amount of charge on the sample surface changes and a shift of a spectral line occurs.
As shown in FIG. 6(a), when a shift of the spectral line occurs depending on a depth of the sample below the surface, the target peak appears outside of a measurement range of a peak or a peak other than the target peak appears within the measurement range. Namely, when detecting a peak of element X, even if the peak of the element X occurs within the measurement range at a depth d.sub.0 and a depth d.sub.1 (or d.sub.0 +.DELTA.d), the peak emerges outside the measurement range at a depth d.sub.2 (or d.sub.1 +.DELTA.d') and a depth d.sub.3 (or d.sub.2 +.DELTA.d"). At the depth d.sub.3, since a peak of an element other than element X appears within the measurement range, the peak of the element other than the element X is measured as a peak of the element X. Therefore, as shown in FIG. 6(b), there is a difference between the apparent composition of the element obtained from the result of analysis and the real composition of the element, preventing accurate analysis.
Moreover, in the elementary mapping analysis using electron spectroscopy, generally, a mapping area is divided into 128.times.128 pixels, for example, and a peak height of the target element of mapping is measured in each pixel. As the number of pixels is great, the peak height in each pixel is measured by detecting intensities of a point E.sub.p of the peak top I.sub.p and one or two points E.sub.b of the background I.sub.b and by obtaining a difference (I.sub.p --I.sub.b) as shown in FIG. 7(a).
In the elementary mapping analysis, if the sample surface has areas of different conductivities, the amount of charge on the sample surface varies depending on locations. Consequently, the shift of spectral line varies depending on locations (P.sub.1 to P.sub.3) of the sample surface. In this case, as shown in FIG. 7(b), the measured peak height is erroneous, preventing accurate mapping analysis.